Circuit Breakers and Substations 4th Year Elec. Power ... - BU

Circuit Breakers & Substations 4th Year Electrical Power Eng. Dep.

Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan 08

Chapter (4)

Transient recovery voltage (TRV)

1. Introduction

Transient is associated with every change of state of a circuit. Transient recovery voltage

thus is the transient across a circuit breaker. Transient recovery voltages are contingent

upon the circuit conditions and parameters of the circuit. Thus making it extremely

essential to study them for application of circuit breakers.

Generally, circuit breakers in a system are applied based on available short circuit

capability at that point in the circuit. But, when circuit is interrupted it results in a TRV,

this has deleterious effects on the circuit breaker. TRV manifests in different way

depending on circuit configuration, hence the object of this report is to study the various

parameters causing and affecting the TRV.

2. Transient Recovery Voltages on Systems

2.1. What is Transient Recovery Voltage?

It is ascertained that TRV is resultant of the change in state of circuit. Thus TRV can be

defined as voltage appearing across a circuit breaker after a switching action. Typical of

every transient TRV also has high amplitude and high frequency. The figure below

shows example of TRV.

Fig. 4.1 Current and voltage during fault clearing


Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan Page 109

TRV is a point by point difference of voltage at the incoming side and at the outgoing

side of a circuit breaker. When a circuit breaker interrupts, incoming side or the side to

bus or supply is connected tries to return to power frequency voltage level and the

outgoing side depending on what is connected also oscillates. The difference between

these voltages is recovery voltage.

TRV is associated with every interruption, but the ones resulting because of interruption

of fault current are the most ominous TRV. Thus the choice of circuit breakers or

introduction of means and methods for safeguarding the circuit and circuit breaker has to

be considered.

2.2. Factors affecting TRV

Transient recovery voltage is affected by various parameters of the system. Prominent

among them are listed below:-

1. Inductance and capacitance in the system

2. Fault current level of the system at point of study of TRV.

3. Bushing capacitance of circuit breakers, voltage transformers etc

4. Number of transmission lines terminating at a bus and their characteristics

impedance.

5. Internal factors of the circuit breaker like the first pole to clear a fault etc.

6. System grounding.

2.3. Types of TRV wave shapes

Transient recovery voltages manifesting across any circuit breaker in a system are seen to

have some typical shapes. Wave shapes are classified into types:

1. Cosine: Observed in case of transformer fed or reactor fed faults.

2. Exponential cosine: Observed when transformer fed faults on breaker terminals with

transmission lines connected on the incoming side of circuit breaker.

3. Triangular or Saw-tooth wave shape: Observed on line side when short transmission

lines are connected.

4. Initial TRV: Observed when bus work of substation is involved.



3. Ratings important for TRV

The above is general rating structure and is provided on nameplate of every circuit

breaker. Rating that are important from the point of view of Transient recovery voltages

depend on the voltage level it is used at and application. IEEE C37.011 is a application

guide for transient recovery voltage for AC high voltage circuit breakers. In addition the

to the peak of the recovery voltage the following ratings are referred to

1. Peak TRV in KV

2. Time to Peak of TRV In μs.

3. Rate of Rise of Recovery Voltage (RRRV)

4. Effects of Transient Recovery Voltages on Circuit Breaker Transient recovery voltages appear on the system for a very short duration of time and

assume very high values. These rapid changes have deleterious effects on the devices in

the system.

4.1 Insulation Failures

TRV is voltage which appears across the circuit breaker on the system when it interrupts

current, and are especially dangerous when short circuits are interrupted. TRV causes the

insulation of the circuit breaker to be stressed and can result in breakdown. The classical

examples of the breakdown of insulation are external flashover between phase and

ground, or between a phase and another. TRV causes the air around to be stressed to

higher levels thus resulting in breakdown.

4.2 Re-ignition and Re-strikes

When the circuit breaker is interrupting a fault, it results in arcing in the interrupting

medium. During the process of interruption the arcing medium is trying to regain its

insulation property. For the interruption to be successful the interrupting medium should

withstand this fast rising recovery voltage.

Thus, there is a race in the interrupting medium to go from conducting state to insulating

state, with the TRV, if the rate of rise of TRV is more than speed with which the medium

returns to insulating state the arcing medium breakdown causing current to continue to

flow in the circuit breaker, if speed of medium is higher the interruption is successful.

This process of establishment of current is called re-ignition and refers to re-ignition of

arc in the circuit breaker.

Re-ignition generally occurs almost immediately after the current zero, is generally

because the arc plasma containing conducting ions reestablishes current.

TRV, if it very high will result in the arcing medium which has returned to insulating

state to stressed. If this field created across the contacts is very high a dielectric

breakdown may take place causing the interrupting medium to become conducting and

carry current. This process of establishing current is called restrike. This generally occurs

about a half cycle later to the current interruption. This phenomenon occurring internally

to the circuit breaker is akin to insulation failure external to the circuit breaker.

5. Calculation of Transient Recovery Voltages of Circuit Breaker At the rated voltage circuit breakers are expected to interrupt rated short circuit current in

a system where the TRV doesn’t exceed the rated TRV envelope. System may be

grounded or ungrounded.



5.1 Envelope Specification and Calculation

TRV rating of circuit breaker is defined by an envelope of rated withstand capability

Parameters that define the envelope are based on the characteristics of actual system.

According to the standard various wave shapes are observed for circuit breakers on the

system, illustrated in section 3. Applicable envelope of TRV depends on the application

and voltage rating of the circuit breaker.

Two envelopes are defined, one for circuit breakers rated 100KV and below and other for

above 100KV.

Circuit breaker is generally used in conjunction with transformers and transmission lines.

As noted earlier the transformer fed faults are known to produce 1-cosine wave, which is

also characteristics of under damped system. Transmission lines, depending upon number

of lines in parallel, are known to add damping to the system and hence produce

exponential waveform. Thus the two envelopes are:-

1. Two parameter envelope used for circuit breakers rated 100KV and below and for

above 100KV, if fault current is less than 30% of rated short circuit current.

2. Four parameter envelope for circuit breakers used above 100KV.

Voltage (E2) can also be calculated using formulae specified in standard and stated as

under

5.2 Factors affecting Transient Recovery Voltage

Transient recovery voltage appearing across a circuit breaker is affected by various

factors like fault current, first pole to clear factor, out of phase switching, system

inductance and capacitance etc…TRV may also be affected by the applications a breaker

is used for.

5.2.1 Faults Current

Calculation of transient recovery voltage Eq.1,2,4 &6 indicates that all forms of TRV are

greatly dependent on the fault current. Short circuit at a point in system depends on the

devices paralleled like the transmission lines, transformers etc. It is observed that TRV

peak increases and time to peak reduces as short circuit reduces. This also causes the

RRRV to increase. Figure below shows the TRV envelopes.

5.2.2 Circuit Parameter

Circuit inductance and capacitance plays a vital role in the TRV peak and time to peak.

Eq.1, 2, 4&6Thus the TRV values can be controlled and reduced using the inductance

and circuit capacitance. Generally system capacitance is considerably less; hence TRV is

controlled by addition of additional surge capacitance.



5.2.3 Out-of-phase Switching

Out of phase switching results in higher values of TRV and is analyzed in a similar way

as the generator circuit breaker analysis. Rated values for out of phase switching are

specified for 900 phase shift.

5.2.4 First Pole to Clear Factors

First pole to clear factors are a function of the system grounding arrangements. It is

defined as ratio, Eq.9.

5.2.5 Applications

TRV depends on the application of circuit breaker. Some important applications are:

1. Transformer fed faults: - Standard capability curve is specified as two parameter

curve. Severe TRV conditions may result if the fault is immediately after

transformer, without appreciable capacitance in between transformer and circuit

breaker. In such cases the TRV peak and RRRV may exceed the values specified in

C37.06-2000; in such circumstances values specified in C37.06.1-2000 are used.

These values correspond to fast time to peak values.

2. Series Reactor limited faults: - Series reactors are used to limit short circuit current

in a line. Use of series reactors on line side can cause high Rate of rise TRV values.

3. Shunt reactor switching: When a shunt reactor is switched circuit breaker interrupts

very small currents. This kind of circuit results in load side oscillating with

frequency of 1/√LC, L being shunt reactor and C being capacitance associated with

it. This can result in very high frequencies sometimes in the range of 1 to 5 KHz.

4. Generator Circuit breakers: - These circuit breakers are required to have special

characteristics as these are installed between the transformer and generators. Faults



associated with this can result in TRV values higher than the values ones given

inC37.04 and C37.06. For system-source faults these can be 3-5 times higher.


Prof. Dr. Sayed A. Ward Eng. Essam M. Shaalan 8



Circuit Breaker Nameplate Catalogue

Circuit Breakers and Substations 4th Year Elec. Power ... - BU

Documents

Transcript of Circuit Breakers and Substations 4th Year Elec. Power ... - BU